Dynamic spin-polarization control of terahertz waves in magnetized plasmas

Controlling terahertz (THz) polarization with high stability and tunability is essential for achieving further progress in ultrafast spectroscopy, structured-light manipulation, and quantum information processing. Here, we propose a magnetized plasma platform for dynamic THz polarization control by exploiting the intrinsic birefringence between extraordinary and ordinary modes. We identify a strong-magnetization, zero-group-velocity-mismatch regime where the two modes share matched group velocities while retaining finite phase birefringence, enabling robust, phase-stable spin angular momentum control. By tuning the plasma length and magnetic field, we realize programmable phase retardation and demonstrate universal single-qubit gates through parameterized unitary operations. Full-wave particle-in-cell simulations validate high-fidelity polarization transformations across the Poincaré sphere and demonstrate the potential for generating structured vector beams under spatially varying magnetic fields. The platform offers ultrafast response, resilience to extreme THz intensities, and in situ tunability, positioning magnetized plasmas as a versatile and damage-resilient medium for next-generation THz polarization control and structured-wave applications.